Selective transfer dehydrogenation of aromatic alcohols on supported palladium

Article abstract of DOI:10.1039/b102463a

Transfer dehydrogenation of various alcohols has been investigated over heterogeneous palladium catalysts using olefins as hydrogen acceptors. Pd/Al₂O₃ together with cyclohexene is the most active and selective system, affording a simple and convenient laboratory synthesis of aromatic ketones in refluxing cyclohexane. Hydrogenolysis-type side reactions can be suppressed by minute amounts of a tertiary amine (selective poisoning of Pd). Aliphatic and cycloaliphatic alcohols are barely reactive under these conditions, a difference which offers the possibility of the selective transformation of aromatic alcohols even in the presence of an aliphatic OH group.

Full text of DOI:10.1039/b102463a

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Selective transfer dehydrogenation of aromatic alcohols on supported
palladium
Csilla Keresszegi, Tamas Mallat and Alfons Baiker*
L aboratory of T echnical Chemistry, Swiss Federal Institute of T echnology, ET H Ho
HCl, CH-8093 Zurich, Switzerland. E-mail: baiker=tech.chem.ethz.ch; Fax: ]411 632 1163
Ž
nggerberg-
Ž
Received (in Montpellier, France) 15th March 2001, Accepted 18th June 2001
First published as an Advance Article on the web 17th August 2001
Transfer dehydrogenation of various alcohols has been investigated over heterogeneous palladium catalysts using
oleÐns as hydrogen acceptors. Pd/Al O together with cyclohexene is the most active and selective system,
2
3
a†ording a simple and convenient laboratory synthesis of aromatic ketones in reÑuxing cyclohexane.
Hydrogenolysis-type side reactions can be suppressed by minute amounts of a tertiary amine (selective poisoning of
Pd). Aliphatic and cycloaliphatic alcohols are barely reactive under these conditions, a di†erence which o†ers the
possibility of the selective transformation of aromatic alcohols even in the presence of an aliphatic OH group.
Oxidation of alcohols to carbonyl compounds is an essential
route in synthetic chemistry. A broad range of homogeneous
and heterogeneous catalysts have been found to be useful for
alcohol oxidation in the past decades.1h6 From an environ-
mental point of view, a highly attractive method is oxidative
dehydrogenation over heterogeneous noble metal catalysts in
aqueous media, using molecular oxygen as the oxidant. The
procedure is particularly useful in carbohydrate chemistry.7h11
Aerobic oxidation of water-insoluble alcohols can be per-
formed in organic solvents but safety problems may hinder its
practical application. Another drawback of the method is the
limited possibility of tuning the selectivity according to special
requirements, for example, di†erentiation between aromatic
and aliphatic OH functional groups.
gen acceptor the acetophenone yield was 35% in 1 h and the
selectivity was also lower (93.7%). We conclude from Table 1
that only cyclohexene and vinyl acetate could accelerate the
dehydrogenation of 1, the other oleÐns rather poisoned the
catalyst. Blocking of active sites by oligomers is a feasible
explanation, though the di†erent reactivity of oleÐns in trans-
fer dehydrogenation reactions may also be due to di†erences
in the adsorption strength relative to that of the sub-
strate.16,17
Among the platinum metals, Pd was by far the most active
in the dehydrogenation of 1. When using cyclohexene in the
presence of various supported Rh, Ru or Pt catalysts, the ace-
tophenone yields were 3% or less after 1 h reaction time. A
comparison of various commercially available supported Pd
catalysts indicates that all catalysts a†orded reasonable reac-
tion rates and selectivity (Table 2). The rate of dehydroge-
nation increased and the selectivity decreased slightly with
increasing catalyst concentration. For the following experi-
ments a catalyst : alcohol mass ratio of 10% has been
employed. The lower selectivity of some catalysts, in particular
with Pd/C (Table 2), is due to the formation of ethylbenzene
by hydrogenolysis of 1. This disproportionation-type side
reaction may be considered as a transfer dehydrogenation
reaction in which the reactant acts both as a hydrogen donor
and an acceptor (Scheme 1). This side reaction reveals a
crucial requirement that the Pd catalystÈoleÐn system has to
Replacing molecular oxygen with
a readily available
unsaturated organic compound as hydrogen acceptor, that is
changing from oxidative dehydrogenation to transfer dehydro-
genation, o†ers the possibility to overcome these limitations.
Transfer dehydrogenation of alcohols over heterogeneous
metal catalysts in the presence of nitrobenzene, phenol,
cyclohexanone and ethylene was proposed many years
ago12,13 but the synthetic potential of the method has barely
been exploited yet. Hayashi and coworkers14,15 reported very
recently that Pd/C in an ethylene atmosphere o†ers good
yields in the dehydrogenation of aromatic alcohols to ketones
at close to ambient conditions, though the reaction times were
usually in the range of days. Here we show that the use of a
liquid oleÐn as the hydrogen acceptor is more convenient and
the reactions are remarkably faster. Application of Pd/Al O
Table 1 E†ect of H-acceptor on the transfer dehydrogenation of 1-
phenylethanol (1) over Pd/Al O a,b
2
3
2
3
with cyclohexene in reÑuxing cyclohexane provides a conve-
nient and highly selective synthesis of aromatic ketones.
H-acceptor : 1
mol ratio
H-acceptor
Yield (%)
Results
Cyclohexene
Cyclohexene
Cyclohexene
Vinyl acetate
Methyl acrylate
5
2
1.2
2
2
2
5
5
2
77
72
69
41
9
InÑuence of catalysts and reaction conditions
Preliminary studies revealed that only a few oleÐns are really
active as hydrogen acceptors in the transfer dehydrogenation
of 1-phenylethanol (1) to acetophenone, a test reaction for the
synthesis of aromatic ketones (Table 1). Over a 5 wt%
Pd/Al O catalyst the reactivity order is cyclohexene [ vinyl
1
-Hexene
6
Cyclopentenec
Styrene
Methyl vinyl ketone
\1
\1
\1
2
3
acetate A methyl acrylate [ 1-hexene. The reaction rate
increased at higher cyclohexene : 1 ratios, though over 90%
acetophenone yield could be achieved in 5 h even with the
lowest ratio applied (1.2) . For comparison, without a hydro-
a Conditions: 0.10 g 5 wt% Pd/Al O , 1.0 g 1, 30 ml cyclohexane, 1
h, 80 ¡C. b The chemoselectivity to acetophenone is always 100%.
c T \ 71 ¡C.
2
3
DOI: 10.1039/b102463a
New J. Chem., 2001, 25, 1163È1167
1163
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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Table 2 Transfer dehydrogenation of 1-phenylethanol (1) to acetophenone with cyclohexene over various supported Pd catalysts and the e†ect
of triethylamine as a selective poisona
Amount of
amineb/wt%
Time/h
Conv. (%)
Select.c (%)
Pd/Al O3
È
1
5
77
93
100
100
2
Pd/CaCO3
Pd/TiO2
È
È
È
1
5
47
79
100
100
1
5
75
93
98.5
97.5
Pd/organopolysiloxane
1
5
84
96
99
99
Pd/C
È
1
1
2
2
1
1
2
1
2
93
91
95
89
93
93
99.5
99.5
99.5
99.5
a Conditions: 0.10 g catalyst (5 wt% metal), 1.0 g (8.2 mmol) 1, 30 ml cyclohexane, alternatively 0.01 or 0.02 g (0.1È0.2 mmol) triethylamine,
alcohol : cyclohexene mol ratio \ 1 : 5, reÑux (80 ¡C). b Relative to the amount of 1. c The other product was ethylbenzene.
fulÐll, namely, that the hydrogen consumption by the oleÐn
should be much faster than the hydrogenation of the reactant.
When necessary, the hydrogenolysis reaction can be selec-
tively poisoned by addition of small amounts of a tertiary
dehydrogenation reaction. Solvents that are reducible and
may act as hydrogen acceptors, such as acetonitrile, prevented
the complete transformation of 1.
amine promoter (e.g., Et N). Organic bases are known to
Scope and limitations of the Pd–cyclohexene system
3
poison hydrogenolysis reactions over Pt metals, while acids
Application of Pd/Al O (without any catalyst pretreatment)
2 3
accelerate them.18,19 An advantage of this solution is that the
overall rate of dehydrogenation of 1 is barely diminished by
the amine (Table 2).
with cyclohexene in reÑuxing cyclohexane seems to be a fast
and convenient laboratory method for the synthesis of aro-
matic ketones. Examples on the application range of the
method are collected in Table 3. Note that all substrates dis-
solved in hot cyclohexane, thus the sometimes low reaction
rates cannot be attributed to solubility problems. Dehydroge-
nation of various secondary aromatic alcohols (1È5) is selec-
tive and a†ords good yields to the corresponding ketones. The
slower reaction in the case of two aromatic rings in the reac-
tant (particularly 6, 7 and 9) is attributed to a too strong
adsorption of the product on the Pd surface. This assumption
is supported by the competitive dehydrogenation of 1 and 6
In alcohol dehydrogenation the metallic surface sites (Pd0)
are much more active than the oxygen-covered Pd or surface
Pd oxides formed by exposing the catalyst to ambient air.9h11
Before the aerobic oxidation of alcohols on Pt or Pd, the cata-
lyst is usually prereduced by hydrogen at room temperature
to form the M0 active sites. The alcohol reactant can also
reduce the oxidized metal but this process is much slower and
incomplete at room temperature.10 Here we found that prere-
duction of Pd by hydrogen at room temperature accelerated
the reaction at low conversion but the yields after several
hours were barely a†ected, compared to the reaction without
this pretreatment. It is important that both the alcohol and
the oleÐn should be added to the catalystÈsolvent slurry
before heating up the reaction mixture. If the slurry is heated
up in the absence of oleÐn, the selectivity drops due to forma-
tion of a mixture of the corresponding carbonyl compound
and hydrocarbon according to Scheme 1.
(Scheme 2). Dehydrogenation of 6 is slow with or without 1,
a†ording less than 1% yield to 1-acetonaphthone. However,
the presence of 6 decelerated the initial rate of transformation
of 1 by a factor of ca. 100. The most likely adsorption mode of
1-acetonaphthone is a Ñat adsorption of the aromatic ring
system and the carbonyl group parallel to the Pd surface,
involving the extended delocalized p-electron system of the
molecule. Apparently, this adsorption mode is strongly
Various solvents may be employed for the reaction. Higher
than 90% yields to acetophenone were obtained in 1È3 h over
Pd/Al O in reÑuxing cyclohexane, heptane and toluene.
2
3
Transfer dehydrogenation of 1 was facile at 80 ¡C or higher
temperatures but rather slow under ambient conditions. Polar
organic solvents that adsorb strongly on Pd retarded the
Scheme 1 Competitive hydrogenation of cyclohexene (H-acceptor)
and 1-phenylethanol (1, reactant) explains the apparent dispro-
portionation of 1 as a typical side reaction during transfer dehydroge-
nation.
Scheme 2 Competitive transfer dehydrogenation of 1 and 6. Condi-
tions: 0.10
g
5
wt% Pd/Al O , 0.5
g
of each alcohol,
2
3
alcohol : cyclohexene mol ratio \ 1 : 2, 30 ml cyclohexane, reÑux
(80 ¡C).
1164
New J. Chem., 2001, 25, 1163È1167

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Table 3 Transfer dehydrogenation of various alcohols to ketones with cyclohexenea
Substrate
Time/h
3
Product
Conv. (%)
92
Select. (%)
100
1
2
3
4
5
5
91
100
50
82
97.5
96
1
5b
21
15
87
100
5
6
88
100
5
0.4
100
7
5
8
46
100
100
1
8
5b
8
9
50c
2c
100
100
7
10
11
12
13
15
5
20
29
87
41
93
50
5
20.5
7.5
5
1
4
5
5
5
1
1
100
100
1
1
6
7
5
5
0
0
È
È
1
a Conditions: 0.10 g 5 wt% Pd/Al O , 1.0 g alcohol, 30 ml cyclohexane, reÑux (80 ¡C), alcohol : cyclohexene mol ratio \ 1 : 2. b 0.30 g 5 wt%
2
3
Pd/Al O , alcohol : cyclohexene mol ratio \ 1 : 5. c Determined by 1H NMR analysis in CDCl .
2
3
3
favored for 1-acetonaphthone compared to 1; the latter pos-
sesses only one aromatic ring and cannot efficiently compete
for the active sites.
Dehydrogenation of primary aromatic alcohols (e.g., 10) to
aldehydes is remarkably slower and also less selective than the
synthesis of aromatic ketones. A more important limitation of
the method is that reducible functional groups, such as aro-
matic halogen (11) or C2C bonds (12) are not stable under the
reaction conditions. Hydrogenolysis or hydrogenation of these
functions by the hydrogen formed in the dehydrogenation of
the CHÈOH function diminishes the selectivity.
A useful feature of the method is that aliphatic and cyclo-
aliphatic primary and secondary alcohols are not reactive.
The same is true for a-functionalized aliphatic alcohols, for
example a-hydroxy esters (16) and 1,2-aminoalcohols (17). An
exception is the dehydrogenation of allylic alcohols, such as
isophorol (13), whose reactions are relatively fast but non-
selective. The poor selectivity to ketones is only partly due to
New J. Chem., 2001, 25, 1163È1167
1165

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acetate or 1-hexene may also be used for the synthesis of aro-
matic carbonyl compounds when lower reaction rates are
acceptable. For example, using these two oleÐns for the syn-
thesis of acetophenone, 97 and 92% yields respectively, were
achieved in 5 h, and the only byproduct was ethylbenzene (for
conditions see Table 3).
The high reactivity of cyclohexene as a hydrogen acceptor
may be astonishing at Ðrst sight, as cyclohexene is widely used
as an H-donor in transfer hydrogenation reactions over sup-
ported Pd catalysts.22h25 In fact, when reÑuxing an
acetophenone : 1-phenylethanol \ 94 : 6 mixture in cyclo-
hexane solvent over Pd/Al O , the ketone : alcohol ratio
Scheme 3 Byproduct formation via isomerization over Pd during
dehydrogenation of allylic alcohols.
2
3
increased or decreased depending on the cyclohexene concen-
tration. When the alcohol : cyclohexene ratio was very low,
the side reaction depicted in Scheme 1. Another, dominant
route may be isomerization via the half-dehydrogenated state
as shown in Scheme 3.20 For example, dehydrogenation of
cinnamyl alcohol (12) a†orded a mixture of 3-phenyl-1-propa-
nol, 3-phenyl-1-propanal and cinnamaldehyde as major pro-
ducts.
The low reactivity of aliphatic (14) and cycloaliphatic (15)
alcohols can be exploited for the selective synthesis of aro-
matic ketones. In the example shown in Scheme 4 a reactant
possessing both aromatic and aliphatic OH groups is mim-
icked by an equimolar mixture of 1 and 14. As expected on
the basis of Table 3, competitive dehydrogenation of 1 and 14
a†orded in 3 h higher than 90% yield to acetophenone while
1
: 40, acetophenone was slowly hydrogenated to 1-
phenylethanol (0.5È1% conversion per hour). The double role
of cyclohexene can explain our observation that increasing the
excess of cyclohexene in transfer dehydrogenation reactions
enhanced the initial rate of alcohol conversion but the Ðnal
yield could not be improved.
We can conclude that transfer dehydrogenation of alcohols
with the Pd/Al O Ècyclohexene system o†ers a facile and con-
2
3
venient synthesis of aromatic ketones. The amount of hydro-
carbon formed by hydrogenolysis of the CHÈOH function can
be minimized by selective poisoning with trace amounts of a
tertiary amine, such as Et N.
3
1
4 remained predominantly unattacked. For comparison, the
Pt metal catalyzed aerobic oxidation of aliphatic alcohols is
not slower than that of aromatic alcohols. As an example, oxi-
Experimental
dation of 1-heptanol and 1-dodecanol on PtO in heptane at
2
6
0 ¡C a†orded 26 and 77% aldehyde yields in 1 and 0.25 h,
Materials
respectively.21 Under the same conditions, oxidation of 10 to
benzaldehyde a†orded 78% yield in 1 h.
Five wt% Pd/C (Fluka, product no. 75992), 5 wt% Pd/Al O
2 3
(
(
Johnson Matthey, product no. 56482), 5 wt% Pd/CaCO
3
5 wt% Pd/TiO (Johnson
Fluka, product no. 76032),
wt% Pd/organopolysiloxane (Degussa,
2
Matthey) and
5
Discussion
product no. 1994-8874) catalysts were used for transfer dehy-
drogenation without any treatment. 1-Phenylethanol (Aldrich,
The Pt metal-catalyzed aqueous phase oxidative dehydroge-
nation of alcohols with molecular oxygen a†ords high yields
to ketones and carboxylic acids under mild conditions. A
drawback of the method is the high activity of Pt and Pd-
based catalysts in the oxidation of almost all types of primary
and secondary alcohols. In contrast, transfer dehydrogenation
over supported Pd catalysts using cyclohexene as the hydro-
gen acceptor allows fast and selective synthesis of aromatic
ketones only; aliphatic and cycloaliphatic primary and sec-
ondary alcohols are barely reactive under the conditions
applied. Application of cyclohexene in reÑuxing cyclohexane is
convenient as the coproduct formed from the hydrogen accep-
tor is identical to the solvent.
[
98%),
DL-6-methoxy-a-methyl-2-naphthalenemethanol
(
(
Acros, 98%), benzyl alcohol (Fluka, 98%), cinnamyl alcohol
Fluka, 97%), 1-dimethylamino-2-propanol (Fluka, 98%), 1-
octanol (Fluka, 99.5%), benzhydrol (Aldrich, 99%), benzoin
Aldrich, 98%), 3,5,5-trimethyl-2-cyclohexene-1-ol (Fluka,
5%), 1-(4-chlorophenyl)ethanol (Aldrich, 98%), 1-(1-naph-
(
9
thyl)ethanol (Fluka, 99%), a-pyridoin (Aldrich, 99%), 1-(4-
methoxyphenyl)ethanol (Aldrich, 99%), ethylmandelate
(Merck, 98%), 1-(4-methylphenyl)ethanol (Lancaster, 97%),
cyclohexanol (Merck, [99%) were used as received. 1-
Indanol (Merck, [98%) was puriÐed by sublimation. Analyti-
cal grade solvents were used as received. Cyclohexene (Fluka,
9
9.5% or Merck, [99%), 1-hexene (Fluka, 98%), methyl acry-
Several liquid oleÐns have been tested together with
Pd/Al O and the conditions have been optimized for cyclo-
late (Merck, [99%), cyclopentene (Aldrich, 99%), styrene
2
3
(
Aldrich, 99%), methyl vinyl ketone (Merck, [95%) and vinyl
hexene, which was the most reactive hydrogen acceptor (Table
). We have to emphasize that other oleÐns such as vinyl
acetate (Aldrich, 99%) were used as H-acceptors. Tri-
ethylamine was a Siegfried Synopharm product (99%).
1
Methods
Catalyst (0.10 g), alcohol (1.0 g), hydrogen acceptor, 30 ml
solvent and Et N (0.01È0.02 g), when used as a catalyst
3
poison, were put in a 100 ml glass reactor. Air was replaced by
Ar and the reactor was put into a preheated oil bath. The
reactions were carried out at reÑux temperature with intensive
magnetic stirring. Alcohol and cyclohexene conversion and
the product distribution were determined by GC analysis
(Thermo Quest Trace 2000, equipped with an HP-FFAP cap-
illary column and FID detector). Yields in the dehydroge-
nation of 8 and 9 were determined by 1H NMR (Bruker
Scheme 4 Competitive transfer dehydrogenation of 1 and 14. Condi-
tions: 0.10 wt% Pd/Al O , 0.5 of each alcohol,
alcohol : cyclohexene mol ratio \ 1 : 2, 30 mol cyclohexane, reÑux
80 ¡C).
Advance DPX300, 300 MHz, in CDCl ). Products were iden-
g
5
g
3
2
3
tiÐed by GC-MS, NMR, and GC analysis of authentic
(
samples.
1166
New J. Chem., 2001, 25, 1163È1167

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1
2
O. Bayer, in Methoden der Organischen Chemie (Houben-W eyl),
Ž
ed. E. Muller, G. Thieme, Stuttgart, 1954, vol. 7/1, p. 160.
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